In electrophotography, a full-color image is formed with toners of four colors: three primary colors of yellow, magenta, and cyan, and black. A light beam reflected from a document is passed through a color separation light transmission filter having complementary colors to toners. The light beam is then directed to an image bearing member so that an electrostatic latent image is formed on a photosensitive layer of the image bearing member. The electrostatic latent image is developed into a single-color toner image, and the single-color toner image is transferred onto a recording medium. This process is successively repeated so that four single-color toner images of yellow, magenta, cyan and black are formed and superimposed on one another on the recording medium. The resulting composite full-color toner image is finally fixed on the recording medium.
In a typical electrophotographic image forming process as described above in which a full-color toner image is formed by superimposing multiple different-colored toner images, toners are required to have specific properties. For example, toners are required to exhibit proper gloss after being fixed on a recording medium while preventing the occurrence of diffuse reflection. Toners are also required to be transparent after being fixed on a recording medium so as not to disturb the color tone of underlying toner layers. Accordingly, toners for full-color printing are required to express greater gloss and transparency than toners for black-and-white printing. In particular, toners for full-color printing are required to express lower viscosity upon application of heat.
Additionally, toners are also required to be fixable at much lower temperatures in accordance with recent demands for energy-saving and high-speed printing. If the low-temperature fixing is achieved by merely reducing the melting point of toner, other problems arise. For example, such a toner may not be stably stored or may degrade a carrier in a two-component developer by contamination.
Generally, a full-color image formed of four color toner images is formed on a white recording medium. When formed on a colored or black paper or a transparent film, the full-color image expresses dull color. In attempting to solve this problem, Japanese Patent Application Publication No. 2006-220694 proposes to form a white background on a colored or black paper or a transparent film with a white toner.
The white toner layer as a background is required to have opacifying power. In other words, the white toner layer is required to completely scatter and reflect incident light to perfectly reproduce white color. When incident light can transmit the white toner layer, it means that opacifying power of the white toner layer is too poor to vividly reproduce a colored toner image formed thereon. Compared to colored or black toners that absorb light, white toners should be more precisely designed to express opacifying power.
To improve opacifying power, various attempts have been made. For example, Japanese Patent Application Publication No. 01-105962 describes a white toner including an aluminum oxide and/or silicon dioxide and Japanese Patent Application Publication No. 2000-56541 describes a white toner including a rutile type TiO2, opacifying power of each of which may be insufficient.
On the other hand, there have been attempts to reproduce metallic texture with toner. For example, Japanese Patent Application Publication Nos. 08-248757 and 08-248719 describe a toner including a mixture of a typical yellow, magenta, or cyan colorant with a fluorescent or transparent powdery agent, for reproducing metallic texture. There may be a concern that the colorant and the fluorescent or transparent powdery agent are not uniformly mixed.
Japanese Patent Application Publication No. 2005-134738 describes a toner including a binder resin and a metallic pigment blended in a dry condition. There may be a concern that the binder resin and the metallic pigment are not uniformly mixed due to the charge difference therebetween. Japanese Patent Application Publication No. 05-289395 describes a metallic toner including a dispersing auxiliary agent which may contaminate and degrade a carrier in a two-component developer.
Japanese Patent Application Publication No. 2009-209367 describes a metallic toner including a metallic material. If the binder resin of this toner does not have sufficient ductility, the outermost surface of a resulting toner layer may not be smooth. Such a non-smooth rough outermost surface diffusely reflects light and prevents the light from reaching the metallic material, resulting in poor luminance and reflectance from the metallic material and poor reproducibility of metallic texture.
The same goes for fluorescent toners such as a toner described in Japanese Patent Application Publication No. 2005-256220. If the binder resin of a fluorescent toner including a fluorescent colorant does not have sufficient ductility, the outermost surface of a resulting toner layer may not be smooth. Such a non-smooth rough outermost surface diffusely reflects light and prevents the light from reaching the fluorescent colorant, resulting in poor luminance and reflectance from the fluorescent colorant and poor reproducibility of fluorescent color. Fluorescent toners are also influenced by the background color tone when its opacifying power is not proper.